Plutonium Signatures in a Dated Sediment Core as a Tool to Reveal Nuclear Sources in the Baltic Sea.

Plutonium distribution was studied in an undisturbed sediment core sampled from the Tvären bay in the vicinity of the Studsvik nuclear facility in Sweden. The complete analysis, including minor isotopes, of the Pu isotope composition (238Pu, 239Pu, 240Pu, 241Pu, 242Pu, and 244Pu) allowed us to establish the Pu origin in this area of the Baltic Sea and to reconstruct the Studsvik aquatic release history. The results show highly enriched 239Pu, probably originating from the Swedish nuclear program in the 1960s and 1970s and the handling of high burn-up nuclear fuel in the later years. In addition, the 244Pu/239Pu atomic ratio for the global fallout period between 1958 and 1965 is suggested to be (7.94 ± 0.31)·10-5. In the bottom layer of the sediment, dated 1953-1957, we detected a higher average 244Pu/239Pu ratio of (1.51 ± 0.11)·10-4, indicating the possible impact of the first US thermonuclear tests (1952-1958).

236U, 237Np and 239,240Pu as complementary fingerprints of radioactiveeffluents in the western Mediterranean Sea and in the Canada Basin (Arctic Ocean).

The aim of this study was to assess the potential of combining the conservatively behaving anthropogenic radionuclides 236U and 237Np to gain information on the origin of water masses tagged with liquid effluents from Nuclear Reprocessing Plants. This work includes samples collected from three full-depth water columns in two areas: i) the Arctic Ocean, where Atlantic waters carry the signal of Sellafield (United Kingdom) and La Hague (France) nuclear reprocessing facilities; and ii) the western Mediterranean Sea, directly impacted by Marcoule reprocessing plant (France). This work is complemented by the study of the particle-reactive Pu isotopes as an additional fingerprint of the source region. In the Canada Basin, Atlantic waters showed the highest concentrations and 237Np/236U ratios in agreement with the estimated values for North Atlantic waters entering the Arctic Ocean and tagged with the signal of European Nuclear Reprocessing Plants. These results may reflect the impact of the documented releases for the 1990s. In the Mediterranean Sea, an excess of 236U presumably caused by Marcoule is reflected in the lower 237Np/236U ratios compared to the Global Fallout signal in all the studied samples. On the contrary, the 239,240Pu profiles were mainly governed by the Global Fallout. The impact of Marcoule as a local source is further corroborated when comparing the temporal evolution of these ratios between 2001 and 2013. The lowest 237Np/236U ratios observed in 2001 at the surface reflect a previous local input that is no longer observed in 2013 as it had been homogenized through the whole water column. This work presents the use of 237Np as a new ocean tracer. A more accurate characterization of the main sources is still needed to optimize the use of 236U-237Np as a new tool to understand transient oceanographic processes.

Presence of 236U and 237Np in a marine ecosystem: The northern Benguela Upwelling System, a case study.

The Benguela Upwelling System (BUS), off the south-western African coast, is one of the four major eastern boundary upwelling ecosystems in the oceans. However, this area has been overlooked in the field of environmental radioactivity. In this work, 236U and 237Np were collected off the coast of Namibia within the northern BUS. Surface seawater exhibited similar 236U and 237Np concentrations, ranging from 3.9·106 to 5.6·106 atoms kg-1 and from 4.6·106 to 8.5·106 atoms kg-1, respectively. The observed inventories in a water column from the continental margin, of (2.10 ± 0.11)·1012 atoms m-2 for 236U and (3.48 ± 0.13)·1012 atoms m-2 for 237Np, were in agreement with the global fallout (GF) source term in the Southern Hemisphere that was the main source of actinides to the region. A pattern was observed in the surface samples, with 237Np concentrations that decreased by 25-30% when moving from inshore to offshore stations, but such an effect could not be clearly discerned in the case of 236U within the data uncertainties. An explanation based on the larger particle reactivity of GF 237Np compared to GF 236U was proposed. Such an effect would have been important at the studied site due to the enhance presence of particles in the continental shelf triggered by the upwelling phenomenon. A value of 1.77 ± 0.20 was obtained for the 237Np/236U atom ratio for the GF source term in the marine environment.

Simple and fast method for the analysis of 236U, 237Np, 239Pu and 240Pu from seawater samples by Accelerator Mass Spectrometry.

A simple and fast radiochemical procedure for the sequential extraction of U, Np and Pu from small-volume seawater samples (<10 L) is presented. The method has been developed and optimized for the final determination of 236U, 237Np, 239Pu and 240Pu by Accelerator Mass Spectrometry (AMS). It is based on the use of 242Pu as tracer for both Np and Pu isotopes. Samples are pre-concentrated by Fe(OH)2 co-precipitation. TEVA® and UTEVA® resins are used in a very simplified way for the final purification of the Pu+Np and U fractions, respectively. The radiochemical yields of the three elements have been investigated in detail by alpha spectrometry (AS) and gamma spectrometry (GS). The obtained results indicate high and robust chemical yields for the three elements and similar ones for Pu and Np. Furthermore, the use of 242Pu as tracer for 237Np is validated by analyzing a reference seawater sample (IAEA-443) by radiometric techniques. We demonstrated that, if chemicals are properly chosen, processed blank levels can be kept at the same level of the extremely low detection limits that can be achieved by AMS (105-106 atoms/sample). The procedure is finally applied for the study by AMS of a reference seawater sample from the Mediterranean Sea (IAEA-418).

Isolation of 236U and 239,240Pu from seawater samples and its determination by Accelerator Mass Spectrometry.

In this work we present and evaluate a radiochemical procedure optimised for the analysis of 236U and 239,240Pu in seawater samples by Accelerator Mass Spectrometry (AMS). The method is based on Fe(OH)3 co-precipitation of actinides and uses TEVA® and UTEVA® extraction chromatography resins in a simplified way for the final U and Pu purification. In order to improve the performance of the method, the radiochemical yields are analysed in 1 to 10L seawater volumes using alpha spectrometry (AS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Robust 80% plutonium recoveries are obtained; however, it is found that Fe(III) concentration in the precipitation solution and sample volume are the two critical and correlated parameters influencing the initial uranium extraction through Fe(OH)3 co-precipitation. Therefore, we propose an expression that optimises the sample volume and Fe(III) amounts according to both the 236U and 239,240Pu concentrations in the samples and the performance parameters of the AMS facility. The method is validated for the current setup of the 1MV AMS system (CNA, Sevilla, Spain), where He gas is used as a stripper, by analysing a set of intercomparison seawater samples, together with the Laboratory of Ion Beam Physics (ETH, Zürich, Switzerland).

Impact of Saharan dust events on radionuclide levels in Monaco air and in the water column of the northwest Mediterranean Sea.

Characterization of atmospheric aerosols collected in Monaco (2004-2008) and in sediment traps at 200 m and 1000 m water depths at the DYFAMED (Dynamics of Atmospheric Fluxes in the Mediterranean Sea) station (2004) was carried out to improve our understanding of the impact of Saharan dust on ground-level air and on the water column. Activity concentrations of natural (210Pb, 210Po, uranium and radium isotopes) and anthropogenic (137Cs, 239Pu, 240Pu, and 239+240Pu) radionuclides and their isotopic ratios confirmed a Saharan impact on the investigated samples. In association with a large particulate matter deposition event in Monaco on 20 February 2004, the 137Cs (∼40 Bq kg-1) and 239+240Pu (∼1 Bq kg-1) activities were almost a factor of two higher than other Saharan deposition dust events. This single-day particle flux represented 72% of the annual atmospheric deposition in Monaco. The annual deposition of Saharan dust on the sea was 232-407 mBq m-2 for 137Cs and 6.8-9.8 mBq m-2 for 239+240Pu and contributed significantly (28-37% for 137Cs and 34-45% for 239+240Pu) to the total annual atmospheric input to the northwest Mediterranean Sea. The 137Cs/239+240Pu activity ratios in dust samples collected during different Saharan dust events confirmed their global fallout origin or mixing with local re-suspended soil particles. In the sediment trap samples the 137Cs activity varied by a factor of two, while the 239+240Pu activity was constant, confirming the different behaviors of Cs (dissolved) and Pu (particle reactive) in the water column. The 137Cs and 239+240Pu activities of sinking particles during the period of the highest mass flux collected in 20 February 2004 at the 200 m and 1000 m water depths represented about 10% and 15%, respectively, of annual deposition from Saharan dust events.

Reassessment of 239Pu on planchets from human urine samples at ultra-trace levels using Aridus-ICPSFMS and AMS.

New analytical methods developed at the facilities here, based on two ultra-sensitive mass spectrometry (MS) techniques, inductively coupled plasma sector field mass spectrometer with a desolvator system (Aridus-ICP-SFMS) and accelerator MS (AMS), have been applied in this work for the reassessment of (239)Pu in alpha spectrometry (AS) planchets corresponding to spiked human urine samples. The obtained (239)Pu minimum detectable activities (MDAs) values by Aridus-ICP-SFMS and AMS were 3 fg (∼6.92 µBq) and 0.4 fg (∼0.92 µBq), respectively, per sample, which are much better than those attainable by AS [50 fg (∼115.3 µBq) of (239)Pu per sample, approximately]. Therefore, it is demonstrated that the MS techniques employed in this work are very powerful tools for internal dosimetry studies in human urine samples, giving excellent results when the reassessment of AS planchets is needed (samples with a Pu concentration below or at the MDA levels measurable by AS). This work is the continuation of an article published in J. Anal. At. Spectrom. 25 (1410-1415) 2010.